Yes, it is possible to edit your own comments or entries after posting in Answers, Discussions, or Notebooks!
Please be aware that you also cannot edit your title!
First, after you have submitted an entry, just hit the "Edit" button.
After you have changed or added the text you wish to, press the Update button!That's it! The post has been edited. There is now a yellow "Edited" icon on the top left of your post to indicate that it has been edited.
Please be careful with what you post in your Notebook, Answers or Discussions as they are visible to everyone and they link back to your profile.
If you want to figure out how to delete entries, you can do so by following the instructions at the bottom of the page here.